I have a "friendpicker" on my app to send an apprequest. It features an option "Select All", so.. when I try to invite 560 friends it doesen't work. Their user ID's don't even fit in the URL.
So, is there any fixed limit for this feature?
I couldn't find any reference to it in the documentation (https://developers.facebook.com/docs/concepts/requests/)
In https://developers.facebook.com/docs/reference/dialogs/requests/ I found the following information regarding the to parameter:
If not (set), the sender will see a multi-friend selector and will be able to select a maximum of 50 recipients.
So, I did a test populating to with 51 and 50 friends. The first gave an explicit error (too much friends) and the second actually passed.
TL;DR
The limit is 50 friends.